Activated carbons are well known to act as catalysts in themselves. For example, activated carbons are known to be useful for various oxidation reactions including oxidation of hydrogen sulfide and that of SO2. It is observed that activated carbons affect such a reaction. Activated carbons as catalysts affect or influence only the rate of the reaction and are hardly changed in themselves by the reaction.
An activated carbon produced from a raw material having a high nitrogen content effectively catalyzes a specific reaction (such as decomposition of hydrogen peroxide) compared with an activated carbon produced from a raw material having a low nitrogen content. Moreover, the activated carbon produced from raw material having a low nitrogen content is also known to increase in a catalytic activity or function thereof when the activated carbon is exposed to a nitrogen-containing compound (such as ammonia) at a high temperature. Nowadays, an activated carbon having a high catalytic activity is produced by carbonizing a material having a high nitrogen content (such as a polyacrylonitrile or a polyamide) at a low temperature or a high temperature and activating the resulting carbonized product. In each case, the activated carbon is produced by heat-treating the raw material at a temperature above 700° C. It is also known that the oxidation of the activated carbon produced from a raw material having a low nitrogen content before or during the exposure to a nitrogen-containing compound is advantageous.
Unfortunately, all of the conventional processes for producing an activated carbon having a catalytic activity have certain disadvantages, thereby limiting an overall usefulness or utility thereof. For example, the raw material having a high nitrogen content (such as a polyacrylonitrile or a polyamide) is expensive and generates a large amount of cyanides and other toxic gas in carbonization. The activated carbon obtained from the raw material having a low nitrogen content requires a violent chemical post-treatment for changing the catalytic capacity significantly. In this regard, a desired catalytic activity is achieved by the sacrifice or reduction of the carbon yield, therefore the resulting activated carbon is inevitably expensive. Further, since the chemical treatment uses a large quantity of a toxic and dangerous chemical (such as nitric acid, sulfuric acid, or ammonia), toxic and dangerous by-products such as SOx, NOx, and a cyanide are produced in significantly large quantities.
Japanese Patent Application Laid-Open Publication No. 60-246328 (JP-60-246328A, Patent Document 1) discloses a process for producing N-(phosphonomethyl)glycine by an oxidation reaction of N-(phosphonomethyl)iminodiacetic acid in the presence of oxygen or an oxygen-containing gas with an activated carbon catalyst in which an oxide has been removed from a surface thereof. This document also discloses that the activated carbon catalyst is obtained by exposing a carbon to an oxidizing agent (e.g., nitric acid) and then pyrolyzing (or thermally decomposing) the carbon in an oxygen-free atmosphere at a temperature of 800 to 1200° C. or by pyrolyzing (or thermally decomposing) a carbon at a temperature of 800 to 1200° C. while passing a gas stream comprising ammonia and an oxygen-containing gas over the carbon. The method for treating the carbon material with the chemical requires use of a toxic and dangerous chemical and produces toxic and dangerous by-products in large quantities, as described above.
There are a large number of conventional arts which deal with the catalytic performance of the activated carbon itself, whereas there are few conventional arts which refer to the relationship between the physical properties of the activated carbon and the catalytic performance thereof in detail except for the following Patent Document 4. The reason includes that the relationship is complicated and difficult to solve due to multiple contributions of various properties of the activated carbon to the catalytic performance.
Japanese Patent No. 2685356 (JP-2685356B, Patent Document. 2) discloses a catalytic-active carbonaceous char for rapidly decomposing hydrogen peroxide in an aqueous solution. This document discloses that the carbonaceous char is produced by oxidation of a bituminous coal or a bituminous coal-like material, in particular, that the carbonaceous char is produced by oxidizing a raw material at a low temperature, exposing the oxidized material to a nitrogen-containing compound (such as urea), heating the resulting material at a high temperature in an inert atmosphere, calcining or activating the resulting material at a high temperature in steam and/or carbon dioxide, and cooling the resulting material in an inert atmosphere. This process is, however, complicated.
Japanese Patent No. 3719756 (JP-3719756B, Patent Document 3) discloses a process for synthesizing a N-(phosphonomethyl)glycine from N-(phosphonomethyl)iminodiacetic acid in coexistence with water, hydrogen peroxide and an activated carbon. This document discloses that a commercially available activated carbon can be used and that the activated carbon is reusable many times. The activated carbon, however, has a low catalytic activity probably due to no optimization of the activated carbon. In addition, recycling of the activated carbon significantly reduces the catalytic activity. Moreover, since this document is silent on an optimizing factor or index of the catalytic activity of the activated carbon, the influence of the activated carbon catalyst relating to the production process of N-(phosphonomethyl)glycine is not revealed.
Japanese Patent Application Laid-Open Publication No. (JP-5-811A, Patent Document 4) disclose an activated carbon as a catalyst for the decomposition of hydrogen peroxide; the activated carbon is made from a protein or a polyacrylonitrile fibrous activated carbon material as a raw material, comprises 1 to 5% by weight of nitrogen, 3 to 30% by weight of oxygen, and 40 to 95% by weight of carbon and has an average pore radius of 15 to 30 Å, and mesopores occupying at least 50% by volume based on the total pore volume. The activated carbon described in Examples of this document comprises 2.1 to 4.1% by weight of nitrogen and 7.6 to 22.8% by weight of oxygen; and that described in Comparative Examples comprises an activated carbon comprising 0.5% by weight of nitrogen and 5.6% by weight of oxygen. These activated carbons, however, not only are insufficient in catalytic activity for decomposing hydrogen peroxide but also sometimes decrease the activity after repetitive use thereof.
Meanwhile, for the disinfection of tap water, a low concentration of a chloramine (chloroamine) is used instead of chlorine. The chloramine includes monochloroamine (monochloramine), dichloroamine, and trichloroamine. Monochloramine is more stable and less vaporizes than chlorine. Moreover, monochloramine does not produce halomethanes even in the presence of methane. Thus use of the chloramine (particularly, monochloramine) is in the process of increasing. In these years, however, many researchers are coming to understand that monochloramine is toxic to an organism, particularly, a freshwater or seawater aquatic organism, and is a causative agent of hemolytic anemia. Accordingly, the development of means for effectively removing the chloramine is required. Moreover, when tap water disinfected with a chloramine is used in an artificial dialyzer, the chloramine permeates through a semipermeable membrane to contact with a blood. Thus, a kidney dialysis unit or other medical units require a high removal of the chloramine.